1. Field of the Invention
The present invention generally relates to an optical information recording medium such as an optical disc for recording information on a track of the recording medium, and more particularly to a multi-layer recording medium having a plurality of recording layers stacked via spacer layers to admit information recording on and information reading from these recording layers.
2. Description of the Related Art
In recent times, information recording media known as DVDs (digital versatile discs) and record/playback systems using such DVDs have become widely known. DVDs are available as DVD-ROMs, DVD-Rs, DVD-RWs, DVD-RAMs, and so on. The DVDs are categorized into the above mentioned types depending upon the recording and/or reading schemes. Referring to FIG. 1 of the accompanying drawings, illustrated is an enlarged schematic view of a recording surface of a DVD-ROM, which is a read-only optical disc. The DVD-ROM has a train of embossed pits, which is a series of concave or convex, on the recording surface as recorded information. A series of concave or convex defines a track. As shown in FIG. 2 of the accompanying drawings, DVD-RW and DVD-R, which are rewritable and recordable optical discs, have pits and grooves formed on the recording surface. The recording surface is made from a dye or a phase-change material. A train of pits (recording indicia) which possess different reflectance is formed on the top of the grooves as recorded information. The pits having different reflectance are created when a beam of light is focused on the grooves as a light spot. The structure of the optical disc depicted in FIG. 2 is referred to as a groove recordation type. Still another optical disc structure is known, which is called a land-groove recordation type optical disc structure as shown in FIG. 3 of the accompanying drawings. A typical example of this type is DVD-RAM, which permits data recording and deleting. In the land-groove type structure, lands and grooves are formed from the phase-change material, and pits (recording indicia) are formed on the lands and grooves.
In order to increase recording density of an optical disc, use of a plurality of recording layers is proposed. An optical disc having more than one recording layer is also one type of the DVDs. Referring to FIG. 4 of the accompanying drawings, a two-layer optical disc of ROM type is illustrated as one example of the multi-layer optical discs. This optical disc has a first recording layer L1 proximal to a pickup, and a second recording layer L2 on the distal side. The first recording layer L1 is located below a transparent cover layer when viewed from the pickup. The first recording layer or upper layer L1 is semitransparent because the beam of light should pass the upper layer L1 and reach the second recording layer or lower layer L2 when retrieving a signal (information) from the lower layer L2. The lower layer L2 is reflective. A spacer layer is interposed between the upper and lower layers L1 and L2 to separate the upper layer L1 from the lower layer L2 with a certain distance. The spacer layer is light transparent (transmissive).
In addition to use of the multi-layer structure, reduction of a diameter of the light spot formed on the recording layer is proposed to increase the recording density of the optical disc. The light spot diameter reduction is achieved by increasing a numerical aperture of an object lens, through which the light beam is radiated to the recording layer to create the light spot on the recording layer. However, spherical aberration occurs in a larger amount as the numerical aperture increases. The spherical aberration increases when the total thickness from the transparent cover layer to a target or desired recording layer, in which information is to be recorded, increases because other recording layer(s) and spacer layer(s) exist in between. The multi-layer optical disc includes a plurality of recording layers stacked via the spacer layers, the recording layers are made from a material which cause reflectance variations upon radiation of the light beam, and the grooves and/or pits are created as a result of localized reflectance change on the recording layers. In such multilayer optical disc, the distance between adjacent recording layers must be reduced in order to accurately focus the light spot having a reduced diameter on the desired recording layer.
The spacer layer thickness should be reduced to shorten the distance between adjacent recording layers. The reduction of the spacer layer thickness, however, causes the light beam recording condition to deviate from the most common optimal condition. Appropriate information recording cannot be expected under such condition. The multi-layer optical disc is an example of multi-layer recording media.
An object of the present invention is to provide a multilayer information recording medium of rewritable type, that can ensure appropriate recording and reading of information into and from each of the recording layers.
In order to accurately record small recording marks in each of recording layers made from a phase change material when recording data on a multi-layer optical disc, it is necessary to optimize recording conditions for the respective recording layers. To this end, a particular area on a target recording layer is used as a write area for trial purpose (trial recording area). Data is first recorded in this trial recording area, and the recorded data is read from the trial recording area and examined. An optimal recording condition for this recording layer is then determined in accordance with a result of the examination. Actual data recording (substantial recording after the trial recording) can be then started with the optimal condition.
It should be assumed here that the optical disc has two layers L1 and L2. Trial data recording into the lower layer L2 is performed with a light beam that has passed the upper layer L1. During this trial data recording, however, there is a problem that a recording condition for the lower layer L2 slightly changes (is slightly influenced) depending upon a track position of the upper layer L1 because the light beam passes the track on the upper layer L1 and reaches the lower layer L2. This is because in reality it is impossible to attach the upper layer L1 onto the lower layer L2 in a perfect manner (with ideal positional relationship between grooves of the upper layer L1 and grooves of the lower layer L2).
In most cases (or practically), there is some misalignment between the upper and lower layers L1 and L2, and the size of misalignment changes with the location on the lower layer L2. For instance, if the center of the upper layer L1 is deviated from the center of the lower layer L2, the misalignment is larger in a peripheral area than in a center area. The practically (possible) optimum recording condition for the lower layer L2 can be obtained if the recording condition determined by the trial recording is an average of a plurality of optimum conditions obtained from various locations of the lower layer L2. However, if the distance between the upper and lower layers is small, positions or shapes of the grooves of the spacer layer relative to the trial recording region influence the recording condition. In particular, as shown in FIG. 5, each groove has a width so that a position of a particular groove of the spacer layer does not change relative to the trial recording region to a certain extent. As a result, the recording condition obtained by the trial recording has some discrepancy from the average optimal condition. Consequently, when the substantial data recording is performed after the trial recording, and the data is recorded in a region having a different groove position condition from the groove position condition of the trial recording, appropriate data recordation cannot be expected.
According to one aspect of the present invention, there is provided a multi-layer information recording medium comprising: a plurality of recording layers, each recording layer having at least one groove and/or at least one pit; at least one spacer layer, each spacer layer being interposed between two adjacent recording layers; an information rewritable region defined in a first recording layer of the recording layers, the first recording layer being a layer in which the information is to be written; and a window region defined in a second recording layer of the recording layers adjacent to the first recording layer for passing a light beam to the information rewritable region and imparting a first diffraction effect to the passing light beam. The second recording layer has the window region and a remaining portion. The first diffraction effect is different from a second diffraction effect imparted from the remaining portion of the second recording layer. Each recording layer may be made from a material of which reflectance changes upon radiation of the light beam. Information is writable in each recording layer as a result of reflectance change made in each recording layer. Even if the distance between the first and second recording layers is small, the window region reduces or minimizes influences of the groove(s) of the second recording layer on the recording into the rewritable region (trial recording region). Size and shape of the window region may be determined to match this goal. Accordingly, accurate recording and reading of data can be performed to the first recording layer. In other words, there is provided a recording medium that has high recording density and good information quality.
Depth of a groove and depth of a pit formed in the window region may be different from depth of another groove and depth of another pit formed in the remaining region of the second recording layer respectively.
A distance between adjacent grooves and a distance between adjacent pits formed in the window region may be different from a distance between adjacent grooves and a distance between adjacent pits formed in the remaining region of the second recording layer respectively.
The multi-layer information recording medium may further include a flat interface formed in the window region.
Each of the plurality of recording layers may include at least one of a land track and a groove track.
Each of the plurality of recording layers may include a medium layer made from a phase change material.